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gh-98963: Add a note to the error for property subclasses without __doc__ #99058

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gh-98963: Add a note to the error for property subclasses without __doc__ #99058

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gh-98963: Add a note to the error for property subclasses without __d…
encukou Nov 3, 2022
2d79454
Fix ReST syntax
encukou Nov 3, 2022
66428bc
Move the note check to an existing test
encukou Nov 3, 2022
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12 changes: 7 additions & 5 deletions Lib/test/test_property.py
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Original file line number Diff line number Diff line change
Expand Up @@ -229,16 +229,18 @@ class PropertySubSlots(property):
class PropertySubclassTests(unittest.TestCase):

def test_slots_docstring_copy_exception(self):
try:
with self.assertRaises(AttributeError) as cm:
class Foo(object):
@PropertySubSlots
def spam(self):
"""Trying to copy this docstring will raise an exception"""
return 1
except AttributeError:
pass
else:
raise Exception("AttributeError not raised")
# gh-98963: A note is added to the exception when we don't have
# a writable __doc__.
notes = cm.exception.__notes__
wanted = "subclasses of 'property' need to provide a writable __doc__"
self.assertTrue(any(note.startswith(wanted) for note in notes), notes)


@unittest.skipIf(sys.flags.optimize >= 2,
"Docstrings are omitted with -O2 and above")
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-11-03-13-54-54.gh-issue-98963.Z3c6M5.rst
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@@ -0,0 +1,3 @@
Add a note to the ``AttributeError`` raised when instantiating a subclass of
:py:class:`property` that does not have a writable ``__doc__`` attribute.
(Creating such a subclass is possible via the C API.)
22 changes: 21 additions & 1 deletion Objects/descrobject.c
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Expand Up @@ -1826,8 +1826,28 @@ property_init_impl(propertyobject *self, PyObject *fget, PyObject *fset,
int err = PyObject_SetAttr(
(PyObject *)self, &_Py_ID(__doc__), prop_doc);
Py_XDECREF(prop_doc);
if (err < 0)
if (err < 0) {
if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
// gh-98963: the subclass doesn't have a __dict__
// (probably it was subclassed using C API).
// Add a note to prevent surprises.
PyObject *type, *value, *traceback;
PyErr_Fetch(&type, &value, &traceback);
PyErr_NormalizeException(&type, &value, &traceback);
if (value) {
PyObject_CallMethod(
value, "add_note", "s",
"subclasses of 'property' need to provide a writable "
"__doc__ attribute (e.g. a __doc__ member or a "
"__dict__) to avoid per-property docstrings being "
"shadowed by the subclass docstring");
// ignore errors while setting the note
PyErr_Clear();
}
PyErr_Restore(type, value, traceback);
}
return -1;
}
}

return 0;
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